1. Field of the Invention
The present invention generally relates to methods and devices used to attach structural members, and more particularly to a method and article for fusion bonding conduits formed of polyolefin materials, such as polyethylene pipes used for natural gas distribution.
2. Description of the Prior Art
Conventional distribution systems for natural gas use polyethylene pipes of varying diameters to transport the gas from suppliers to end users. There is a continual problem regarding leakage in such pipe systems due to inadequate joining of pipe sections, or due to ruptures which may occur in the pipes after placement in the ground. Such joining, and repair of ruptures, was at one time accomplished by the use of various adhesives (see, e.g., U.S. Pat. No. 3,826,521), but this was generally unacceptable due to the difficult adherence characteristics of polyethylene which would result, after a long period of time, in cracks in the cured adhesive or separation of the adhesive from the pipe surface. Indeed, bonding nearly anything to polyethylene is very difficult, as recognized in U.S. Pat. No. 3,143,364.
Another technique for fitting pipes together involves the use of electro-fusion couplers (see, e.g., U.S. Pat. No. 4,486,650). These couplers overcome the adherence properties of the polyolefin compositions by being formed of the same thermoplastic material as the conduits to be joined. An electrical resistance heating element is molded into the coupler, along the inner surface thereof, and upon energization of the element by direct current, it raises the temperature of the thermoplastic material in both the coupler and the adjacent pipes which have been fitted within the coupler. The material of the coupler and the pipes then flows together and fuses as it cools. As with other prior art techniques, however, electrofusion couplers often do not create a complete seal and the entire process must sometimes be repeated. Electrofusion couplers are also relatively expensive and consequently have not gained widespread use.
Fusing of the pipes may also be accomplished by the use of special films which may be applied to the joint or rupture and heated to flow with the pipe material. For example, Patent Cooperation Treaty Application US88/02909 depicts a method of applying a conductive, polymeric gasket to a pipe joint, and heating the gasket by passing electrical current therethrough. A similar process is disclosed in Japanese Patent Application (Kokai) No. 54-58777 which uses a dielectric heater to melt a conductive sheet which has been wrapped around the pipes. One disadvantage of these techniques is that they require a relatively high conductivity, further requiring the use of metallic or carbonaceous fillers. The inclusion of these fillers in the bonding matrix causes the bond to be more brittle which can lead to cracking over a long period of time. The concern over the use of these fillers in the bonding material is highlighted by the industry (ASTM & Plastic Pipe Institute) requirement that metallic particles be present in an amount not greater than 1% by weight. An equally significant disadvantage relates to the power supply required for these fusion bonding techniques. Dielectric heating requires special electrode pairs and equipment that is both heavy and bulk, which makes it much more difficult to use in the environment of a gas pipe trench. It would, therefore, be desirable and advantageous to devise a method for bonding polyolefin pipes which would both simplify the bonding process and provide a bonding material which would be acceptable to the gas distribution industry.